In the prior art, there are the wiring substrates for mounting the electronic component such as the semiconductor chip, or the like. In one example of such wiring substrates, the build-up wiring is formed on both surface sides of the core substrate made of a glass epoxy resin, or the like respectively. In recent years, the miniaturization and the higher performance, etc. of the electronic component devices are demanded, in order to respond to such demands, a reduction in thickness of the core substrate of the wiring substrate is advanced.
A related art is disclosed in Japanese Laid-open Patent Publication No. 2011-181630, Japanese Laid-open Patent Publication No. 2010-232418, and Japanese Laid-open Patent Publication No. 2010-10329.
As explained in the column of preliminary matter described later, when the core substrate of the wiring substrate is thinned, sufficient rigidity cannot be obtained. As a result, there exists such a problem that a twist or a warp is caused in the wiring substrate by the heating process, or the like in the steps of manufacturing the wiring substrate.